1. Field of the Invention
This invention relates to a process for preparing lithographic printing plate bases made of aluminum or an aluminum alloy. More specifically, it relates to a process for preparing lithographic printing plate bases which possess a surface superior in hydrophilicity and water retention properties and which has a good adhesion to a photosensitive coating layer provided on the aluminum or aluminum alloy sheet and an outstanding resistance to printing.
2. Description of the Prior Art
One of the base materials for lithographic printing plates which have heretofore been used is aluminum sheets which, in most cases, are grained or roughened prior to use by mechanical polishing, chemical etching, electrolytic etching or similar technique in order to improve their hydrophilicity for dampening water used in printing procedure, water retention properties and adhesion to a photosensitive coating layer provided thereon.
The surface topographies of the grained aluminum sheets greatly contribute to the plate making performance of the coated sheets as printing plates. Therefore it is highly important to control the surface topographies of the grained aluminum sheets.
For instance, those grained surface in which relatively fine, shallow pits are present densely are suitable as printing plate bases for use on a proof press for which a good image reproducibility and a high resolving power are required. On the other hand, those grained surface in which the pits are deep and uniform in diameter (uniform microscopically) are suitable as printing plate bases for use on a regular press for which a marked water retention and a high resistance to printing are required.
Among various graining techniques, particularly electrolytic etching techniques have attracted attention in recent years because it makes possible the production of a wide variety of grained surface varying from a relatively fine, shallow grain to a deep and uniform one, as compared with mechanical polishing techniques including ball polishing and brush polishing and chemical etching techniques.
In the elctrolytic etching process, an aluminum sheet is generally immersed in a suitable electrolyte solution and electrolyzed with direct or alternating current to grain the surface.
The most well-known electrolyte for this purpose is hydrochloric acid. However, when hydrochloric acid is used in electrolytic etching of an aluminum sheet, it is difficult to produce deep grains which are uniform in microscopic topography (e.g., topography observed under magnification to 100 to 1,200 diameter on a microscope or the like.) Therefore, particularly when used as printing plate bases for use on a regular press, these bases are not always satisfactory in adhesion properties of the coating layer in the image area and in resistance to printing, although they are superior in water retention properties and in removability of the coating layer in the non-image area during developing process.
Accordingly, there is a continuing need for a lithographic printing base which is superior in water retention and resistance to printing.